Micromachined cantilevers, as used in atomic force microscopy (AFM), hold promise in chemical-sensing applications, and may become the basis for specialized, ultraminiature, ultrasensitive sensors for detection of specific target chemical species such as chemical compounds, bioactive agents, or toxins. There are increased demands for miniaturized chemical sensors that provide sensitive chemical detection, quality control of materials processing, and measurements of small or limited quantities of a chemical or biological material. Atomic force microscopy, also called scanning force microscope (SFM), is a part of a larger realm of microscopy called scanning probe microscopy. It is being used to solve processing and materials problems in a wide range of technologies affecting the chemical, biological, energy, electronics, telecommunications, automotive, and aerospace industries. Some of the materials being investigated include synthetic and biological membranes, thick and thin film coatings, cerarics, composites, glasses, metals, polymers, and semiconductors. Phenomena such as lubrication, abrasion, corrosion, adhesion, friction, cleaning, polishing, plating and etching are being studied with AFM.
AFM is a method of measuring surface topography on a scale typically from a few angstroms or less to a hundred micrometers or more. The technique involves imaging a sample through the use of a probe or tip suspended from one end of a microcantilever. A surface is probed with the tip, and the interaction between the tip and sample is measured. Physical topography, surface chemistry, charge density, magnetic properties, local temperature and other surface properties can be analyzed.
With an AFM that operates in the contact mode, the surface of a sample is raster scanned by the AFM tip, which is mounted onto the end of a flexible cantilevered beam. The deflection of the cantilever due to tip-surface interaction detects changes in the sample surface. Samples can be analyzed in air, liquids or vacuum. Biological samples have been difficult to scan using contact mode because they are weakly secured to the surface and can be easily scraped off.
With non-contact methods, a tip may be held several nanometers above the surface using a feedback mechanism that measures surface-tip interactions on the scale of nano-Newtons or less. Variations in tip height are recorded while the tip is scanned repeatedly across the sample, producing a topographic image of the surface.
The non-contact mode has been preferred for imaging, because forces on the sample are much lower than in contact mode and less likely to cause damage to soft samples. The cantilever is oscillated close to its resonant frequency at a small distance on the order of one to ten nanometers above the surface. Long-range attractive forces induce changes in the oscillation amplitude, frequency and phase of the cantilever. A constant distance is maintained during scanning.
As an alternative between contact and non-contact modes, the cantilever of the AFM oscillates at its resonant frequency like the non-contact mode, but like the contact mode, it gently taps the surface during a small fraction of its oscillation period, which helps reduce damaging lateral forces. Data may be collected from interactions with surface topography, stiffness, and adhesion as variations in tip height are recorded while the tip is scanned repeatedly across the sample. A three-dimensional topographic image of the surface may be produced from the data. The usual method for displaying the data is to use color mapping for height, for example, black for low features and white for high features.
An AFM operating in a vibrating or tapping mode may use a piezoelectrically actuated microcantilevered probe. Typically, the probe is a micro-electrical-mechanical-system (MEMS) device, micromachined from bulk silicon with a piezoelectric film patterned along a portion of the microcantilever. At the free end of the cantilever is a tip with nanometer-scale radius, optimally shaped to probe the sample surface. The microcantilever is displaced by voltage applied to the piezoelectric actuator, resulting in a controlled vertical movement of the tip. Control electronics drive the microcantilever while simultaneously positioning it vertically to track the sample topography and follow the surface features. A macro-scale position actuator may be used to null the position of the cantilever, following the topology of the sample as the probe is scanned over the surface.
Tapping mode AFM has become an important tool, capable of nanometer-scale resolution on biological samples. The periodic contact with the sample surface minimizes frictional forces, avoiding significant damage to fragile or loosely attached samples.
An AFM can be operated with or without feedback control. Applying feedback can help avoid problems with thermal drift and avoid damage to the tip, cantilever or sample during sample measurement. The AFM may be placed into a constant-height or constant-force mode, which is particularly useful when samples are quite flat and a high-resolution image is needed. With the constant-height mode, the force applied to the sample increases with cantilever deflection, which may result in damage to the tip or the sample. When operated in a constant-force mode of operation, the positioning piezoelectric element moves the sample up and down in response to any changes in detected force. The tip-sample separation is adjusted to restore the force to a pre-determined value. In the constant force mode, however, variations in sample compressibility may yield inconsistent and inaccurate results.
Deflections of the AFM tip can be measured using optical detection methods. Optical sensing of cantilever deflections using a light source with a scanned optical assembly that guides light onto the AFM cantilever, and a photodetector to measure reflected light is disclosed by Prater, et al., in “Scanning Stylus Atomic Force Microscope with Cantilever Tracking and Optical Access,” U.S. Pat. No. 6,032,518, issued Mar. 7, 2000.
Oscillating probe tips may be operated in an intermittent mode against a sample to determine surface topology and ascertain physical aspects of the sample surface. An atomic force microscope in which a probe tip is oscillated at a resonant frequency and at a constant amplitude setpoint while scanned across the surface of a sample is disclosed by Elings, et al., in “Tapping Atomic Force Microscope with Phase or Frequency Detection,” U.S. Pat. No. 5,519,212, issued May 21, 1996. Tapping mode AFM operation in liquids is discussed by Rogers, et al., in “Tapping Mode Atomic Force Microscopy in Liquid with an Insulated Piezoelectric Microactuator,” Review of Scientific Instruments, Vol. 73, No. 9, Sepember 2002, pp. 3242-3244.
Self-sensing cantilevered AFM tips with a layer of zinc oxide (ZnO) partially covering a silicon cantilever for combined sensing and actuating in the fundamental and higher order resonant modes is discussed in “Contact Imaging in the Atomic Force Microscope Using a Higher Order Flexural Mode Combined with a New Sensor,” Appl. Phys. Lett. 68 (10), 4 Mar. 1996, pp. 1427-1429. Lee, et al., also describe a self-sensed SFM tip with a PZT thin film on a silicon dioxide cantilever in “Self-Excited Piezoelectric PZT Microcantilevers for Dynamic SFM—With Inherent Sensing and Actuating Capabilities,” Sensors and Actuators A72 (1999), pp. 179-188. Minne, et al., describe a cantilever with a tip operating at a higher resonant mode in “Vibrating Probe for a Scanning Probe Microscope,” U.S. Pat. No. 6,075,585, issued Jun. 13, 2000. Minne, et al. also describe a cantilever with a piezoelectric drive and a piezoresistive sense in “Cantilever for Scanning Probe Microscope Including Piezoelectric Element and Method of Using the Same,” U.S. Pat. No. 5,742,377, issued Apr. 21, 1998.
In non-contact AFM, a cantilevered probe with a piezoelectric film is described by Miyahara, et al., in “Non-Contact Atomic Force Microscope with a PZT Cantilever Used for Deflection Sensing, Direct Oscillation and Feedback Actuation,” Applied Surface Science 188 (2002), pp. 450-455.
Chemical sensing based on frequency shifts of microcantilevers treated with a compound-selective substance is disclosed by Thundat, et al., in “Microcantilever Sensor,” U.S. Pat. No. 5,719,324, issued Feb. 17, 1998. Oscillating silicon nitride cantilevered beams coated with a thin gold film have been used to detect mercury vapor in air due to changes in cantilever resonant frequency and stress levels induced in the gold overlayer as described by Thundat, et al., in “Detection of Mercury Vapor Using Resonating Microcantilevers,” Appl. Phys. Lett. 66 (13), 27 Mar. 1995, pp. 1695-1697. An uncoated microcantilever can be used for chemical sensing by exciting charge carriers into or out of surface states with discrete photon wavelengths as disclosed by Thundat, et al., in “Uncoated Microcantilevers as Chemical Sensors,” U.S. Pat. No. 6,212,939, issued Apr. 10, 2001. Attempts at DNA sequencing and detection using an AFM is described by Allen in “Method and Apparatus for DNA Sequencing Using a Local Sensitive Force Detector,” U.S. Pat. No. 6,280,939, issued Aug. 28, 2001.
Current methods for detecting AFM cantilevers generally use optical methods, though these systems require comparably high power, need alignment, have limited resolution, and are prone to drift because of the size of the optical path and the need to retain all parts and optical elements securely coupled to each other. Current systems typically require external lighting for sample illumination and setup, and are not very compact because of the long optical path, the need to have the photodetector at an ample distance from the sample, and constrained viewing and positioning systems for optical alignment. Sample testing in liquids such as water or saline solution presents additional difficulties for optical sensing due to aberrations and refraction of the light beam traversing the fluid.
Piezoresistive sense methods are more compact and may have more resolution than optical systems, though they can self-heat and cause drift. Furthermore, piezoresistive sensing typically consumes large portions of available power when used in a portable device. Piezoelectric devices are generally not selected for static bending measurements due to leakage and charge decay issues of many piezoelectric films. Chemical sensing with static or quasi-static cantilever bending and piezoelectric sensing in response to chemical exposure is difficult due to the minute deflection changes of the cantilever curvature and the tendency of detection systems to drift. A preferred system would not require static curvature measurements.
A compact, beneficial method for sensing chemicals with an AFM cantilever would not require alignment of an external laser and photodetector. Without the laser and photodetector, a chemical-sensing cantilevered system could be more compact. The method would require less power, and generate less heat than other methods. Cantilevers would be small and light, for rapid detection of minute concentrations of target chemicals and biomaterials. Bulky scanning mechanisms would be eliminated. The enclosure would be small, and capable of operating in liquid or gas. Feedback would be readily attained, allowing for parallel probe operation for increased reliability and increased imaging bandwidth. Affects of confounding chemical species such as moisture would be minimized or eliminated. The effects of added mass due to absorption and the effects of surface reactions resulting in altered cantilever stiffness or modifications to stresses on the surface of the cantilever could be detected and sorted to aid in chemical species identification and specificity.
The preferred method would detect quasi-static bending of a cantilever from absorption of material preferentially disposed on the cantilever, could be used with optical, piezoresistive, piezoelectric and other techniques for sensing, and could be used to ascertain cantilever bending without requiring DC or static stability.
It is, therefore, an objective of the present invention to provide a method and system for chemical sensing using cantilevered probes, and to overcome the obstacles and deficiencies described above.